On the growth mechanisms of nanocrystalline diamond films

Nanocrystalline diamond/amorphous carbon (NCD/a-C) composite films have been deposited by microwave chemical vapour deposition from methane-rich CH4/N-2 mixtures and characterized carefully by a variety of methods with respect to morphology and structure, composition, crystallinity, and bonding environment. The films consist of diamond nanocrystals of 3-5 nm diameter, which are embedded in an amorphous carbon matrix of 1-1.5 nm width. The matrix is a mixture of sp(2) (20-30%) and sp(2) bonded carbon and contains about 20% of hydrogen, mostly bonded to sp(3) carbon atoms. New experiments are reported carried out to study the nucleation and the growth on a microscopic and macroscopic scale. No differences to standard deposition of polycrystalline diamond (PCD) with respect to the nucleation step were observed, i.e. in order to achieve a high nucleation density an appropriate substrate pretreatment is required. In contrast, there are strong differences in the growth mechanisms of NCD and PCD, respectively. Most importantly, there is a high rate of secondary nucleation observed in NCD deposition. On a macroscopic scale, this leads to a spherulitic growth of NCD films. The microscopic mechanisms leading to this high rate of secondary nucleation are discussed in terms of the deposition parameters leading to NCD growth, namely gas phase constituents and composition, pressure, substrate temperature, and bias voltage. Defect formation under the special conditions of NCD deposition is identified as a major reason.